Planet of the Bugs: Evolution and the Rise of Insects by Scott Richard Shaw Read Free Book Online

lines time to drift apart; natural selection insured that the remaining cells were indeed real survivors.
    The global ice age, which may have lasted for millions of years, is indicated in the earth’s rocks by banded iron layers; these layers formed when iron accumulated in the oceans then precipitated into sediments. The rocks are capped with a calcium carbonate layer, indicating that the ice age ended abruptly with a period of global warming when continental minerals were washed into the seas, stimulating a worldwide flush of bacterial growth. Oxygen was ejected back into the atmosphere, and the world teemed once again with eukaryotic cells.
    We might suppose that such a near-death experience might have been the necessary jolt to set life along a more complex pathway. But that does not seem to be the case. Bacterial cells resumed their old pattern of floating around for tens of millions of years. Then, about 850 million years ago, continental drift brought the land masses into an unfavorable configuration near the equator, and again the earth was cast into a planetary deep freeze. Glacial ice approached the equator, and the chill lasted for millions of years. Finally, the ice was broken, and life enjoyed a brief reprieve. But this time a cycle was established, and between 850 and 590 million years ago the earth experienced not just one but at least four global ice ages. The most recent of these, the great Varanger ice age, lasted 20 million years, from 610 to 590 million years ago; scientists have dubbed it the time of the “snowball earth.”
    As the last snowball earth came to an end and the last global glaciers retreated toward the poles, life reassembled into multicellular clusters. Soon after that, abundant early animals appeared in the “Cambrian explosion.” What finally stimulated such dramatic changes in life forms, after billions of years of single-cellular domination? What happened most notably is that atmospheric oxygen levels finally rose to levels approximating our modern atmosphere. Potential oxygen toxicity drove cells into clusters for safety, but at the same time an energetic system existed to motivate animal life: aerobic respiration. Animals live more complex and energetic lives than bacteria because oxygen forced them to do it, and oxygen enabled them to do it. This process of oxygen levels affecting the evolution of life, the history of changes in oxygen levels, and the geological evidence for all this is thoroughly covered in Nick Lane’s popular and entertaining book,
Oxygen, The Molecule That Made the World
. So, there’s no need for me to repeat it all here.
    After the last snowball earth, another very important event happened: continental drift accelerated and the Precambrian continents were dramatically reconfigured. The continental drift rate during the Cambrian has been estimated to be about ten times faster than the average rate since then. This was important for two reasons. First, it brought continental land masses back near the poles in a surprisingly rapid fashion. This stabilized the planet by reestablishing the cycle of weather that keeps ice ages more moderate. Some land masses have been near a pole, one way or the other, since that time, keeping modern earth out of the severe “snowball” phase. Second, for the Cambrian animals it was a bonanza, because rising sea levels and rapidly drifting continents meant lots of shorelines and more shallow marine communities with abundant mineral sediments. The earth was spinning faster during the Cambrian and the moon was closer, so tidal forces on shallow marine communities caused rapid pulses of nutrient flow. The time was ripe for rapid evolution of animal life.
    Skeletons in the Cambrian Closet
     
    Some of the earliest Cambrian animal action took place in shallow marine sediments. Among the oldest Cambrian fossils are “trace fossils” that do not show the actual animal, but animal tracks. These are abundant fossilized burrows,